I have the same misgivings as you about this design. If one imagines the blanket salt system draining, the neutrons from the core are exposed to less absorptions from the blanket salt, but also some additional moderation from the graphite that lines the core.
I think, for safety's sake, it is important to bake into any design the loss of any salt system. I think the two fluid annular design can accomplish this, particularly if surrounding the outside of the pipes there is a material that acts as a neutron absorber. I'm guessing, (correct me if I'm wrong) that there will be an insignificant amount of diffusion of the neutrons to the absorber if the blanket salt is in the annulus. If the blanket salt were to drain, however, that would expose the absorber to greater flux, causing the reactor to go subcritical.
Also, I think the graphite issues - geometry changes that affect reactivity and the need for constant replacement - are problematic.
In short, the design has features that are worth studying, but I'm concerned that the apparent design goal of optimizing breeding has traded off against safety issues. This is something that is ultimately undesirable, particularly at the outset.